Need to Accurately Measure Time in Space? Use a COMPASSO

Author: Andy Tomaswick | Published on: December 19, 2024

Telling time in space is difficult, but it is absolutely critical for applications ranging from testing relativity to navigating down the road. Atomic clocks, such as those used on the Global Navigation Satellite System (GNSS) network, are accurate, but only up to a point. Moving to even more precise navigation tools would require even more accurate clocks. There are several solutions at various stages of technical development, and one from Germany's DLR, COMPASSO, plans to prove quantum optical clocks in space as a potential successor.

The Challenges with Current Timekeeping Technologies

There are several problems with existing atomic clocks – one has to do with their accuracy, and one has to do with their size, weight, and power (SWaP) requirements. Current atomic clocks used in the GNSS are relatively compact, coming in at around 0.5 kg and measuring 125 x 100 x 40 mm, but they lack the required accuracy. In the highly accurate clock world, terminology attributes a "stability" of 10^-9 over 10,000 seconds to these clocks. While this sounds absurdly precise, it is simply not good enough for a more demanding GNSS.

Alternatives, such as atomic lattice clocks, are more accurate, achieving stability down to 10^-18 stability for just 10,000 seconds. However, their size is unfathomable for satellite applications, measuring approximately 0.5 x 0.5 x 0.5 m and weighing hundreds of kilograms. Given satellite space and weight constraints, those are way too large to be adopted as a basis for satellite timekeeping.

The ESA and NASA Innovations

To find a middle ground, the European Space Agency (ESA) has developed a technology roadmap aiming to improve clock stability while keeping devices small enough to fit on satellites. A notable example in this roadmap is a cesium-based clock cooled by lasers and combined with a hydrogen-based maser, a microwave laser. NASA is not falling behind; their mercury ion clock has been orbitally tested for a year, showing promising results.

COMPASSO: The Innovative System

COMPASSO aims to surpass existing technologies by incorporating three key technologies: two iodine frequency references, a “frequency comb”, and a “laser communication and ranging terminal”. The mission is expected to be launched to the International Space Station (ISS), where it will operate continuously for two years, keeping time consistently. Over this period, the accuracy of its timekeeping will be compared to existing alternatives.

The Role of Lasers in Timekeeping

Lasers play a crucial role in the COMPASSO system. The iodine frequency references exhibit distinct absorption lines of molecular iodine, enabling precise frequency reference for the frequency comb. The frequency comb, a specialized laser, possesses an output spectrum appearing like comb teeth at specific frequencies. These frequencies can be tuned to align with the iodine reference, allowing for drift correction in the comb.

Phase Locking and Stability Transfer

The frequency comb provides a method for phase-locking a microwave oscillator, a standard component in atomic clocks. This transfer of stability allows the iodine frequency reference to stabilize the frequency comb, which subsequently stabilizes the microwave oscillator and, thus, the atomic clock. Additionally, the laser communication terminal is used to transmit frequency and timing information back to ground stations while it remains active.

Mission Timeline and Future Implications

COMPASSO commenced its development in 2021, with documentation on its details and initial breadboarding prototypes released this year. The mission's launch is slated for 2025, with the goal of enhancing global timing accuracy, potentially revolutionizing navigation abilities internationally.

Further Reading

Interdisciplinary studies surrounding timekeeping technology detail the significance of advancements. For expansive literature on this subject, consider the following research:

• Kuschewski et al – COMPASSO mission and its iodine clock: outline of the clock design
• Atomic Clocks Separated by Just a Few Centimetres Measure Different Rates of Time. Just as Einstein Predicted
• Deep Space Atomic Clocks Will Help Spacecraft Answer, with Incredible Precision, if They’re There Yet
• A New Atomic Clock has been Built that Would be Off by Less than a Second Since the Big Bang

Conclusion

As global navigation and timing technology progress, projects like COMPASSO will provide essential advancements necessary for increasing accuracy in time measurement across various applications. By relying on innovative technologies, aerospace agencies are setting the groundwork for future explorations and everyday navigation enhancements.

Lead Image: Benchtop prototype of part of the COMPASSO system.    Credit – Kuschewski et al

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